The invention relates to a data transmission system having a light-emitting transmitter, a light-receiving receiver and a data transmission channel based on incoherent light. The invention also relates to a method for transmission in such a data transmission system.
With a so-called free space transmission, in particular a data transmission in the close range with incoherent light by a rapid modulation of the optical power of an optical light source, it is disadvantageous that data rates above 40 MBit/s cannot be realized using simple modulation methods, like for instance the on-off keying (OOK). The reason for this is that the minimal modulation bandwidth of the transmitter and of the receiver can result in a blurring of the data signal and thus in a superimposition of transmitted symbols. IR (infrared) LEDs in the data transmission standard IrDA (Infrared Data Association) or also fluorescent tubes can be used as optical sources in such a data transmission system to transmit visible light for instance. White light LEDs are in principle also possible as optical sources. Their continually improving efficiency nevertheless results in a restriction in the useable modulation bandwidth, thereby impeding the achievement of high data rates.
A higher spectral efficiency can, as is known, impede a bandwidth restriction in a transmission channel. A multistage modulation can be used here for instance. With this, the symbols are embodied for so long that no significant superimposition of the transmitted symbols takes place. To increase the data rates, symbols with a higher value can be transmitted instead of a logical “1” and a logical “0”, e.g. logical “2”, “3”, “4” etc. The amplitude is then greater by the factor 2, 3 or 4 for instance than with a logical “1” (Pulse Amplitude Modulation, PAM). Within digital imaging, gray scales are the counterpart to this multistage modulation.
With the afore-cited pulse amplitude modulation, 2 bits can therefore be transmitted in each symbol, namely “00”, “01”, “10” or “11”. This means that with N different amplitudes/gray scales log2 (N), more bits can be transmitted per symbol than with the modulation method on-off keying. If the transmission speed in the case of on-off keying is restricted to 25 MBit/s for instance, then 125 MBit/s can be transmitted with 25=32 amplitudes/gray scales.
It is common to all multistage modulation methods that the output received by the receiver and also the signal-to-noise ratio has to be greater than in the case of on-off keying, so that the amplitudes/gray scales can be clearly distinguished from one another. A further common feature is that the linearity of the data transmission system has to be improved.
In connection with the data transmission by way of a wireless infrared transmission channel, the use of a pre-equalization of the data signal to be transmitted by way of the transmission channel is known (cf. K. L. Sterckx and J. M. H. Elmirghani, “On the Use or Pre-distortion Equalization in Infrared Wireless Communication Connections” IEEE International Conference on Communications, Vol. 7, Pgs. 2166-2170, 2001). Here the special instance of an untargeted transmission, i.e. a transmission during which the transmitter and the receiver can “see” one another, determines why a significant portion of the light radiation output by the transmitter is reflective diffusely. Such a reflection can be produced by walls and/or fixtures/fittings in a room. With the data transmission described in this document, a bandwidth restriction exists in the free space channel, i.e. the “blurring” of the transmitted symbols materialize as result of single or multiple reflections on the walls for instance. The data transmission system described in this document is disadvantageous in that the pre-equalization depends on the transmission function of the free space channel. For instance the transmission function is dependent on whether and which furniture is present in the room of the data transmission system. This however results in the pre-equalization having to be adjusted to the respective environment and also to changes, which, provided it is actually possible, requires a return channel of the receiver in order to adjust the pre-equalization to the change in the free space channel. As a result, a data transmission system made from the system perspective is too complex and too expensive.